Quasi-equilibrium theory: Difference between revisions
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|acronym=([[QET]]) | |acronym=([[QET]]) | ||
|def=Statistical theory of unimolecular reaction rates (isomerization or fragmentation) of molecular species that assumes that reaction occurs from the ground electronic state of the reactant regardless of its mode of formation, at a rate determined by the free statistical redistribution of internal energy into those vibrational and rotational modes that must be activated sufficiently to reach the appropriate transition state for the specified reaction to occur. The QET applies to isolated molecular species in the absence of collisions, as for [[ion]] fragmentations in a [[mass spectrometer]]. Early formulations assumed the reactant contained s identical harmonic oscillators of frequency ν, with a transition state corresponding to just one critical oscillator containing at least m quanta corresponding to the critical energy for reaction ε* = mν. A further assumption that ''m'' >> ''s'' leads to an ultra-simple expression for the unimolecular rate constant at total internal energy ε: k(ε) = ν[( ε $$$ ε*)/ε](''s''$$$1). Elimination of most of these simplifying assumptions followed development of the [[Rice-Ramsperger-Kassel-Marcus theory|Rice-Ramsperger-Kassel-Marcus]] ([[RRKM]]) theory. | |def=Statistical theory of unimolecular reaction rates (isomerization or fragmentation) of molecular species that assumes that reaction occurs from the ground electronic state of the reactant regardless of its mode of formation, at a rate determined by the free statistical redistribution of internal energy into those vibrational and rotational modes that must be activated sufficiently to reach the appropriate transition state for the specified reaction to occur. The QET applies to isolated molecular species in the absence of collisions, as for [[ion]] fragmentations in a [[mass spectrometer]]. Early formulations assumed the reactant contained s identical harmonic oscillators of frequency ν, with a transition state corresponding to just one critical oscillator containing at least m quanta corresponding to the critical energy for reaction ε* = mν. A further assumption that ''m'' >> ''s'' leads to an ultra-simple expression for the unimolecular rate constant at total internal energy ε: k(ε) = ν[( ε $$$ ε*)/ε](''s''$$$1). Elimination of most of these simplifying assumptions followed development of the [[Rice-Ramsperger-Kassel-Marcus (RRKM) theory|Rice-Ramsperger-Kassel-Marcus]] ([[RRKM]]) theory. | ||
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|ref=revised from | |ref=revised from | ||
Latest revision as of 14:17, 14 March 2024
| IUPAC RECOMMENDATIONS 2013 |
| Quasi-equilibrium theory (QET) |
|---|
| Statistical theory of unimolecular reaction rates (isomerization or fragmentation) of molecular species that assumes that reaction occurs from the ground electronic state of the reactant regardless of its mode of formation, at a rate determined by the free statistical redistribution of internal energy into those vibrational and rotational modes that must be activated sufficiently to reach the appropriate transition state for the specified reaction to occur. The QET applies to isolated molecular species in the absence of collisions, as for ion fragmentations in a mass spectrometer. Early formulations assumed the reactant contained s identical harmonic oscillators of frequency ν, with a transition state corresponding to just one critical oscillator containing at least m quanta corresponding to the critical energy for reaction ε* = mν. A further assumption that m >> s leads to an ultra-simple expression for the unimolecular rate constant at total internal energy ε: k(ε) = ν[( ε $$$ ε*)/ε](s$$$1). Elimination of most of these simplifying assumptions followed development of the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. |
| Related Term(s): |
| Reference(s):
revised from IUPAC. Compendium of Chemical Terminology, 2nd ed. (the Gold Book). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. with additional information from H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, H. Eyring. Proc. Natl. Acad. Sci. USA 38, 667 (1952). (http://dx.doi.org/10.1073/pnas.38.8.667 ) T. Baercor, P. M. Mayer. J. Am. Soc. Mass Spectrom. 8, 103 (1997). (http://dx.doi.org/10.1016/S1044-0305(96)00212-7 ) |
| From Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013); DOI: 10.1351/PAC-REC-06-04-06 © IUPAC 2013. |
